Set xparse variables to lowercase

Question

A while ago, I asked for a method to comfortably define several variables and the best answer came up with xparse and its key-value pairs. I now have two related issues, which I would like to discuss here.

Note: It's probably very useful to check the original post before going on, so you know my xparse setup.

---

Issue 1

I now tried to use the \lowercase{} function on these "variables".

\lowercase{\GetInvoiceData{FirstName}}

This results in the output Unknown, if I just call \GetInvoiceData{FirstName}, everything works just fine.

So my question is, how to print the value of an xparse key in lowercase.

---

Issue 2

I added the % as proposed by Christian Hupfer in the comments, which solves this issue.

Furthermore I wanted to print my whole name in lowercase letters and joined together (the first name in a thin font, the second name in a bold type) like so:

johndoe

My current code looks like this

\newcommand{\headertext}[2]
{%
    \color{white}%
    {%
        \Huge{%
            %\lowercase{%
                \textlight{#1}\textbold{#2}%
            %}%
        }%
    }%
}
% later in the code
\headertext{\GetInvoiceData{FirstName}}{\GetInvoiceData{SecondName}}

Note that \textlight{} and \textbold{} only define which font should be used:

\newcommand{\textlight}[1]
{%
    % Normally defines another font
    {#1}
}
% Same for \textbold{}

As you see, \lowercase{} is currently commented out because of the Unknown-issue, but the output of this is:

John Doe

So the font selection works well, but why is there a space between the two values? And how can I get rid of it?

---

MWE

\documentclass{article}
%% %%%%%%%%%%%%%%%%%%%%%%%
%% Variables Configuration
%% %%%%%%%%%%%%%%%%%%%%%%%
\RequirePackage{xparse}
\ExplSyntaxOn
\prop_new:N \g_invoice_prop

\cs_new:Nn \invoice_store_property:nn {
    \prop_gput:Nnn \g_invoice_prop {#1} {#2}
}

%Defining key-value interface
\keys_define:nn {invoice} {
    FirstName       .code:n = {\invoice_store_property:nn {FirstName}{#1} },
    LastName        .code:n = {\invoice_store_property:nn {LastName}{#1} },
}

\NewDocumentCommand{\StoreInvoiceData}{+m}{
    \prop_gclear:N \g_invoice_prop% Clearing the property list
    % Set the keys
    \keys_set:nn {invoice}{#1}
}

\NewDocumentCommand{\GetInvoiceData}{m}{
    \prop_if_in:NnTF \g_invoice_prop {#1} 
    {% True
        \prop_item:Nn \g_invoice_prop {#1}% Extract the key #1 from the property list
    }{% False
        Unknown
    }
}

% Check if the field is given and execute #2 for true case, otherwise #3
\NewDocumentCommand{\IfInvoiceFieldGivenTF}{m+m+m}{%
    \prop_if_in:NnTF \g_invoice_prop {
        #1% Check to check
    }{% True #2
        #2
    }{% False
        #3
    }
}% End of \IfInVoiceFieldGivenT

% Do something only if #1 is given 
\NewDocumentCommand{\IfInvoiceFieldGivenT}{m+m}{%
    \prop_if_in:NnT \g_invoice_prop {
        #1% Check to check
    }{% True #2
        #2
    }%
}% End of \IfInVoiceFieldGivenT


\ExplSyntaxOff
\StoreInvoiceData{
    FirstName   =   {John},
    LastName    =   {Doe},
}
%% %%%%%%%%%%%%%%%%%%
%% Font Configuration
%% %%%%%%%%%%%%%%%%%%
\newcommand{\textlight}[1]
{%
    % Normally defines another font
    {#1}%
}
\newcommand{\textbold}[1]
{%
    % Normally defines another font
    {\bfseries#1}%
}
\newcommand{\headertext}[2]
    {%
        \color{black}
        {%
            \Huge{%
                \lowercase{%
                    \textlight{#1}\textbold{#2}
                }
            }
        }
    }
\begin{document}
    \headertext{\GetInvoiceData{FirstName}}{\GetInvoiceData{LastName}}
\end{document}

Comment out \lowercase{} and you'll see, that the name is printed correctly. When using the function, it only prints Unknown.